Cell membranes

• all cells are surrounded by membranes. In eukaryotic cells, many of the organelles are surrounded by membranes too
• • cell-surface membranes surround cells
  • barrier between the cell and its environment
  • controlling which substances enter and leave the cell
  • partially permeable - let some molecules through but not others
• substances can move across the cell-surface membrane by diffusion, osmosis or active transport
• sometimes called the plasma membrane
• the membranes around organelles divide the cell into different compartments - act as a barrier between the organelle and the cytoplasm
• e.g. the substances needed for respiration (like enzymes) are kept together inside a mitochondrion by the membrane surrounding the mitochondrion
• membranes within cells are also partially permeable and control what substances enter and leave the organelle
• RNA leaves the nucleus via the nuclear membrane (nuclear envelope). DNA is too large to pass through the partially permeable membrane so it remains in the nucleus
• the basic structure of all membranes is pretty much the same:

  • composed of lipids (mainly phospholipids), proteins and carbohydrates (attached to proteins or lipids)
• 1972 - fluid mosaic model was suggested to describe the arrangement of molecules in the membrane
• in the model, phospholipids form a continuous, double layer (bilayer) which is fluid bc the phospholipids are constantly moving
• proteins are scattered through the bilayer,

  • include channel proteins and carrier proteins
  • allow large molecules and ions to pass through the membrane
  • receptor proteins on the cell-surface membrane allow the cell to detect chemicals released from other cells
  • the chemicals signal to the cell to respond in some way
  • e.g. the hormone insulin binds to receptor proteins on liver cells, which tells the cells to absorb glucose
• some proteins are able to move sideways through the bilayer, while others are fixed in positions
• some proteins have a carbohydrate attached - glycoproteins
• some lipids also have a carbohydrate attached - glycolipids
• cholesterol molecules are also present within the bilayer
• phospholipid molecules form a barrier to dissolved (water-soluble) substances
• phospholipids have a head and tail

  • head = hydrophilic (attracts water)
  • tail = hydrophobic (repels water)
• the molecules automatically arrange themselves into a bilayer - heads face out towards the water on either side of the membrane.

  the centre of the bilayer is hydrophobic so the membrane doesn't allow water-soluble substances (like ions and polar molecules) to diffuse through it.

  small, non-polar substances (e.g. carbon dioxide) and water can diffuse through the membrane
• Cholesterol - gives the membranes stability

  • type of lipid that is present in all cell membranes (expect bacterial cell membranes)
  • fits between the phospholipids
  • binds to the hydrophobic tails of the phospholipids causing them to pack more closely together
  • restricts movement of the phospholipids making the membrane less fluid and more rigid
• cholesterol helps to maintain the shape of animals cells (don't have cell walls)

  particularly important for cells that aren't supported by other cells e.g. red blood cells which float free in the blood
• cholesterol also has hydrophobic regions - so is able to create a further barrier to polar substances moving through the membrane
• cell membranes are affected by temperature - affects how much the phospholipids in the bilayer can move which affects membrane structure and permeability
• temps below 0

  • phospholipids don't have much energy - can't move very much
  • packed closely together and the membrane is rigid
  • but channel proteins and carrier proteins in the membrane denature (lose structure and function) increasing permeability of the membrane
  • ice crystals may form and pierce the membrane, making it highly permeable when it thaws
• temps between 0-45

  • phospholipids can move around and are not packed as tightly together
  • membrane is partially permeable
  • as temp increases - phospholipids move more bc they have more energy - increases permeability of the membrane
• temps above 45

  • phospholipid bilayer starts to melt (break down) and membrane becomes more permeable
  • water inside the cell expands, putting pressure on the membrane
  • channel proteins and carrier proteins in the membrane denature so they cannot control what enters or leaves the cell
  • increases the permeability of the membrane